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        <p>如果直接调用SQLite数据库，很容易引入错误，假如Activity中有错别字，就会将无效输入插入到数据库中。可以通过引入一个名为Content Provider的概念，作为数据库和活动之间的一个层。使用其有很多好处，可以用它确保输入数据库中的数据是有效的。介绍它，必须介绍URI、UriMatcher、Content Resolver。<a id="more"></a></p>
<h3 id="Content-Providers存在的必要性"><a href="#Content-Providers存在的必要性" class="headerlink" title="Content Providers存在的必要性"></a>Content Providers存在的必要性</h3><p>Content Providers可帮助我们管理对有结构的数据集的访问，它可以作为数据源和UI代码之间很好的抽象层。这个抽象层可以添加数据验证，帮助我们修改数据存储的方式，而UI代码始终保持不变。</p>
<p><img src="https://i.imgur.com/PGBjPre.png" alt=""></p>
<p>Content Providers可以与其他框架类完美结合。</p>
<p>为了更清楚说明这一点，继续讨论宠物列表。当宠物列表添加或被删除，我们希望UI显示最新消息时，我们每次都必须调用query和insert方法。取代这种繁琐工作的方法就是，利用一个名为Cursor Loader框架类。每当有宠物添加到列表或被删除时，列表会借助Cursor Loader始终处于更新状态。</p>
<p>因为Cursor Loader会在数据发生变更时自动进行检查，并在确定发生了数据变更后自动更新列表。Cursor Loader能与我们前面讨论的ListView和Cursor Adapter协作。从而在列表中显示所做的更改。而实现Cursor Loader需要用到Content Provider。所以Content Provider和Cursor Loader一起为我们省了很多工作，使我们不需要在发生数据更改时，一次次手动执行查询。</p>
<p>它还能与主屏幕小部件搭配使用。这些部件的名称为同步适配器，能将我们的数据同步到云，并为应用提供搜索建议。</p>
<p>另一个重要的优势是，分享数据。当应用中存在文本数据或文件时，其他应用是无法访问的。不过，可以通过Content Provider将我们的数据暴露给其他应用。这样，其他应用可以使用querty、insert、update和delete方法与provider交互从而访问我们的信息。</p>
<p>当然，这样做存在一些隐患。因为可能不希望某个应用访问和修改所有的应用数据，而且遇到恶意应用或许会删掉整个数据库表。所以，向其他应用暴露数据还需谨慎，处理不当可能会有危险。不过，开发者不同担心，Content Provider能以非常安全的方式管理数据访问，如果想要访问其他应用，提示用户授权访问数据的特定权限Content Provider可以帮助予以执行，而未获得权限的其他应用将无法访问数据。</p>
<p>总之，Content Providers有三大优点：</p>
<ul>
<li>作为数据源和UI代码之间的抽象层，可以很好地进行数据验证；</li>
<li>帮助我们与其他框架类完美协助；</li>
<li>帮助我们轻松与其他应用分享数据。</li>
</ul>
<p>一个应用向其他应用请求特定的数据，它会参考contacts contract，并创建特定内容URI请求。内容URI唯一标识一组数据，类似于Web URI地址唯一标识网络上的唯一网站。</p>
<p><img src="https://i.imgur.com/VVm5Srq.png" alt=""></p>
<p>Android系统根据我没在内容URI中传入的信息，知道Google+要向contacts app通信。给定这些信息，它会判断哪个content provider和应用可以处理请求。</p>
<p>Android上还有一个东西，叫做content resolver，它以URI为输入，来决定应该由哪种content provider处理我们的请求，判断秘诀就在于内容主机名。内容主机名是URI的一部分，它会告诉content resolver应由哪个provider来处理请求，内容主机名与网络URL的部分相似。</p>
<p><img src="https://i.imgur.com/iNXhq2J.png" alt=""></p>
<p>所以，可以得到所有这些其他应用实际上直接与content resolver通信，而非provider。content resovler会暴露这些方法：query()、insert()、update()和delete()。它对provider调用适当的方法，后者相应地调用适当的provider方法对数据库进行操作。</p>
<p>在一个应用中的实现：</p>
<p><img src="https://i.imgur.com/NTDjimI.png" alt=""></p>
<p>其中Content Provider是一个抽象类。</p>
<h3 id="URI"><a href="#URI" class="headerlink" title="URI"></a>URI</h3><p>与Provider交流时，需要告诉它两件事，第一，你想要执行什么操作；第二，在这个操作中使用什么数据。</p>
<p>URI代表同一资源标识符。它可以标识出我们感兴趣的资源的名称、位置。URL是URI的子集，URL代表统一资源定位符，它会给出某个文件或数据在网站上的具体位置。内容URI主要用来标识Provider中的数据，它可以指向数据库的某个部分，比如单个行、单个表或一组表，它也可以指向文件，比如文本文件、照片或其他媒体文件。</p>
<p><img src="https://i.imgur.com/smL8VqY.png" alt=""></p>
<p>内容URI由三个部分组成：</p>
<ul>
<li>scheme：<code>content://</code>，这是Android应用中URI的标准开头。</li>
<li>主机名：最重要的部分，它指定我们要使用的content Provider，称为内容主机名，它的主要作用是指定我们要进行通信的content Provider类。</li>
<li>数据类型：指定了我们要执行操作的数据。</li>
</ul>
<p>存在两种：</p>
<ul>
<li>对于整个表进行操作：这时数据类型为表名，如下：</li>
</ul>
<p><img src="https://i.imgur.com/1XJKbpK.png" alt=""></p>
<ul>
<li>对某行进行操作：使用另一种URI，在表名后面跟上具体行的ID，如下：</li>
</ul>
<p><img src="https://i.imgur.com/AHCV6sg.png" alt=""></p>
<p>汇总：</p>
<p><img src="https://i.imgur.com/rbG8DM8.png" alt=""></p>
<h3 id="URI-Matcher"><a href="#URI-Matcher" class="headerlink" title="URI Matcher"></a>URI Matcher</h3><p>Provider为了决定如何处理Content Resolver的请求，会使用URI matcher。URI matcher会确定传递给它的URI属于哪种类型。然后，我们可以用这个信息决定要进入的分支，比如，第一分支要对整个表进行操作；另一个分支，对某个行禁止操作。这样，我们就确切地知道了需要操作的数据库表。</p>
<p><img src="https://i.imgur.com/a0QZ6j0.png" alt=""></p>
<p><img src="https://i.imgur.com/j2Lj591.png" alt=""></p>
<p>URI Matcher用于将URI与整数代码相匹配，使用URI matcher需要实现的两个步骤：</p>
<ul>
<li>第一步是用Content Provider可接受的URI模式设置UriMatcher，并向每个模式分配一个整数代码；</li>
<li>第二步需要调用一个UriMatcher.match(Uri)方法传入一个Uri，返回对应的整数代码（前提有与之匹配的模式），如果不匹配它会给我们提示。</li>
</ul>
<p>实现示例：</p>
<p><img src="https://i.imgur.com/e8FpgWv.png" alt=""></p>
<p>其中CONTACTS和CONTACTS_ID是全局常量。sUriMatcher是一个静态全局UriMatcher对象。</p>
<pre><code>private static final UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
</code></pre><p>:</p>
<pre><code>private static final int PETS = 100;
private static final int PETS_ID = 101;

private static final UriMatcher sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);

static {
    sUriMatcher.addURI(PetContract.CONTENT_AUTHORITY,PetContract.PATH_PETS, PETS);

    sUriMatcher.addURI(PetContract.CONTENT_AUTHORITY,PetContract.PATH_PETS + &quot;/#&quot;, PETS_ID);
}
</code></pre><h3 id="查询query"><a href="#查询query" class="headerlink" title="查询query()"></a>查询query()</h3><p><img src="https://i.imgur.com/NMAq3qI.png" alt=""></p>
<p>query方法共分为三部分：</p>
<ul>
<li>首先，需要获得数据库对象；</li>
<li>将URI发送到URI matcher;</li>
<li>帮我们决定选择哪条分支。</li>
</ul>
<p><img src="https://i.imgur.com/Vnx0lcK.png" alt=""></p>
<p>具体实现代码：</p>
<p><img src="https://i.imgur.com/Oib8Er5.png" alt=""></p>
<p>代码：</p>
<pre><code> @Override
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs,
                    String sortOrder) {
    SQLiteDatabase database = mDbHelper.getReadableDatabase();

    Cursor cursor;

    int match = sUriMatcher.match(uri);
    switch (match){
        case PETS:
            cursor = database.query(PetContract.PetEntry.TABLE_NAME, projection,selection,selectionArgs,null, null,sortOrder);
            break;
        case PETS_ID:
            selection = PetContract.PetEntry._ID + &quot;=?&quot;;
            selectionArgs = new String[] {String.valueOf(ContentUris.parseId(uri))};
            cursor = database.query(PetContract.PetEntry.TABLE_NAME, projection, selection, selectionArgs,
                    null, null, sortOrder);
            break;
            default:
                throw new IllegalArgumentException(&quot;Cannot query unknown URI + &quot; + uri);
    }

    return cursor;
}
</code></pre><p>该方法的调用：</p>
<pre><code>Cursor cursor = getContentResolver().query(PetContract.CONTENT_URI,
                                          projection,
                                          null,
                                          null,
                                          null);
</code></pre><h3 id="插入insert"><a href="#插入insert" class="headerlink" title="插入insert"></a>插入insert</h3><p><img src="https://i.imgur.com/MxUs9g9.png" alt=""></p>
<p>实现步骤：</p>
<p><img src="https://i.imgur.com/cwuH84y.png" alt=""></p>
<p>实现代码：</p>
<p><img src="https://i.imgur.com/nyUlqb1.png" alt=""></p>
<h3 id="更新Update"><a href="#更新Update" class="headerlink" title="更新Update"></a>更新Update</h3><p>根据来自UriMatcher的结果，PETS和PET_ID case均支持。在PETS case中，调用者想要按照selection和selectionArgs 更新 pets 表中的多个行。 在PET_ID case中，调用者想要更新特定宠物。要写入数据库中的新值包含在传入方法的 ContentValues 对象中。</p>
<p><img src="https://i.imgur.com/6kVNAVI.png" alt=""></p>
<p>实现流程：</p>
<p><img src="https://i.imgur.com/PcSVuUW.png" alt=""></p>
<h3 id="删除delete"><a href="#删除delete" class="headerlink" title="删除delete"></a>删除delete</h3><h3 id="gettype"><a href="#gettype" class="headerlink" title="gettype"></a>gettype</h3><p>getType(Uri uri) 方法。此方法的用途是返回描述输入 URI 中存储的数据类型的字符串。该字符串为 MIME 类型，也称为内容类型。</p>

      
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